We now turn to the other two regions within the Orion Nebula complex. In M43, 50% of [C ii] emission arise in the region we have defined as the shell. However, this emission is likely contributed to by a significant emitting background. About 25% stem from the inner region. Of this emission, a spectral analysis shows that only 25% arise from the background, while 5% stem from the expanding shell and 70% from the H ii gas filling the bubble. Faint [C ii] emission can be observed outwards of the shell, accounting for 25% of the total emission. The percentages in the FIR emission are similar, slightly more (60%) stemming from the shell region. H\(\alpha \)  emission is concentrated in the inner H ii region. Emission at 24 \(\mu \) m is generally connected to either temperature fluctuations in very small grains or to dust in the ionized gas heated by, for instance, trapped Lyman \(\alpha \)  radiation [1]}, [2]}, [3]}. Also CO emission is dominated by the shell region and follows its morphology. The face-on shell seen toward the H ii region does not exhibit CO emission, hence we conclude that the expanding shell is not very massive and that the edge-on shell structures are sculpted within the molecular background. In [4]} we estimated an expanding shell mass of only \(8\,M_{}\) , which amounts to 1.5% of the total gas mass in M43. This compares to 5% of [C ii] emission stemming from the face-on shell. If we adopt a shell mass of \(1000\,M_{}\)  in M42, 10% of the gas mass account for 25% of the [C ii] emission, which is an equal relative mass to luminosity ratio as in M43.
